In the field of nuclear medicine, an image is produced either by radioisotopes that are absorbed within the patient organs and that emit gamma radiation therefrom, or by X-ray radiation that is produced by an X-ray source that transmits its radiation via the patient body. In X-ray and gamma ray imaging systems that operate at a high-energy spectrum (short wavelengths), such as the systems used for medical imaging, an imaging component, such as a lens, is not available. Thus a collimator is used to acquire the image.
Gamma rays emitted from the patient organs or X-ray radiation that passes through the patient's body suffer from a scattering effect known as Compton scattering. The scattered radiation causes image distortions and appears as background noise in the acquired image.
Accordingly, it is highly important to discriminate the scattered radiation and not use it for image reconstruction. There is a correlation between the scattering angle of the radiation scattered by Compton scattering and the energy loss of the scattered photons in this radiation: The larger the scattering angle of the radiation scattered by Compton scattering, the greater the energy loss of the scattered photons. The original energy of the photons is a known value determined by the type of the radioisotopes that produces the photons. Accordingly, using only photons that did not suffer energy loss, or which suffered relatively low energy loss, in comparison to their original energy for the image reconstruction may assure that the image is constructed with no or very little distortion from scattered photons.
Measuring the photons' energy may be done by the well-known technique of single photon counting. In a single photon counting technique, a radiation detector measures the radiation photon-by-photon, when for each photon its energy is measured. A histogram, known as a spectrum, is built by the computer of the imaging system and presents the number of photons versus their energy. The energy span is divided into discrete values of energy ranges known as energy channels.
Two very common types of single photon counting radiation detectors are indirect conversion and direct conversion radiation detectors. The indirect conversion detector is based on a combination of a scintillator (such as NaI scintllator) with photomultipliers. The direct conversion detector is a semiconductor detector such as CdZnTe (CZT).
The energy resolution of the detector is a critical parameter in rejecting the scattered photons: The higher the energy resolution of the detector, the higher the detector efficiency. The efficiency of the detector goes up with its energy resolution since the number of photons (out of the number of photons impinging on the detector) that appear in a relatively narrow energy window around the energy of the photon emitted by the radioisotope and used for the image reconstruction increases with the energy-resolution of the detector.
The photons that are used for the image reconstruction are the photons with measured energy that falls in the energy range (the width of the energy window) around the energy of the spectrum peak (peak channel) corresponding to the energy of the radioisotope as measured by the detector. The peak channel is the energy channel with a value equal to that of the channel for which the spectrum has a maximum of counts or events. The energy is measured in units of energy channels that are proportional to the energy in KeV units. Usually, the width of the energy window is determined by the imaging system as a fraction or percentage of the energy of the radioisotope energy. However, the position of the peak channel of the radioisotope on the energy axis, as measured by the detector, may be affected by the following parameters:                1. The total gain of the system, consisting of the product of the electronic gain of the system and the detector gain; and        2. The system offset, usually caused by the offset of the electronic unit of the system when the detector does not contribute to the offset.        
Due to the electronic offset mentioned above, the energy position of the measured peak-channel of the spectrum is not necessarily proportional to the energy of the radioisotope. Thus the width of the energy window used for the imaging cannot be derived only from the peak position of the spectrum.
In addition, the radiation detector used in single photon counting is typically a pixilated detector where each of the pixels of the detector is connected to a different electronic amplification channel. Thus, each of the detector pixels may measure the position of the energy peak at a different energy since it may have a different gain that is further amplified and shifted by the different gain and offset of the respective electronic channel.